An increasing demand for accuracy in such diverse fields as geophysical measurement (e.g., land surveying) and long distance communication (e.g., electro-optic communication networks) has created a need for a laser transmitter capable of accurately projecting a beam to a distant reflector. Often, such systems are in continuous use in remote locations. It is, therefore, essential that the subsystems of the transmitter, particularly the resonator cavity, have a long lifetime, high reliability and minimum power consumption. Additionally, the frequent use of such systems in mobile applications makes it desirable for the principal subsystems, particularly the resonator cavity, to be small in volume and mass.
In its simplest form, the optical feedback loop for a resonator cavity of an optically pumped laser transmitter is an active laser medium, such as a crystalline rod, made optically resonant by a pair of reflecting surfaces placed at either end of the medium. Both of the reflecting surfaces and the laser medium are usually attached by mounting fixtures to a common optical bed for stability. The reflectors need not be plane mirrors, but must be aligned so that multiple reflections occur. Consequently, such a resonator cavity is very sensitive to changes in alignment between the laser medium and the mirrors. A slight tilt of one mirror, for example, creates a misalignment between the medium and the mirror which subjects the resonator cavity to a high loss of energy. Typically, a misalignment of three-tenths of a milliradian in the resonator cavity can prevent operation of a laser transmitter. Additionally, the likelihood of misalignment losses is increased by two techniques commonly used to increase the radiated power of a resonator cavity.
In one technique, the degree of resonance achieved by an optical cavity is enhanced with a Q-switch, an active component. A Q-switch operates to retard stimulated emission by interrupting oscillation in the resonant cavity. Then, when a very high level of inversion is reached in the laser medium, the Q-switch is triggered to suddenly restore the resonant cavity. A Q-switch is susceptible to damage by divergence of the laser beam; therefore, its performance, and, in turn, that of the laser transmitter, is degraded by either misalignment or dimensional instability within the resonator cavity. The second technique, mode-locking, forces the different wavelengths (i.e., "spectral modes") oscillating in the resonator cavity to interact so that they all have the same relative phase. This means that a mode-locked, optically pumped laser propagates a series of short pulses separated by a time lapse of one round trip transit time of the beam inside the resonator cavity. Consequently, any variation in the distance between the reflecting surfaces, such as may be caused by dimensional instability, will create interference and reduce the peak power.
Previous efforts to improve dimensional stability and alignment insensitivity have included the use of concave mirrors, opposite corner cube prisms tilted against correlative Brewster angles, and internal reflection prisms arranged with either parallel or obliquely crossed rooflines, as reflective surfaces.
A total internal reflection prism may have either a real or a virtual roofline. A total internal reflection prism with a real roofline, sometimes referred to as a Porro prism, is a prism having a right triangular cross-section. The edge running along the right angle is the "roofline." The prism provides total reflection of light incident to a face opposite a 45.degree. angle when the light is sufficiently parallel so that all of the light strikes the reflecting face outside of the critical angle. A total internal reflection prism with a virtual roof-line is a prism with a trapezoidal cross-section having 45.degree.-45.degree.-135.degree.-135.degree. included angles; in effect, a real roofline prism from which the roofline edge has been truncated. A virtual roofline prism reflects light incident to a face included between 45.degree. and 135.degree. angles when the light is sufficiently parallel such that all of the light strikes the reflecting face outside the critical angle. Laser energy is coherent, and therefore a light beam emitted by a lasing medium can be totally reflected by a total internal reflection prism, albeit with a small translation within the plane of, but normal to, the light beam. There is an 180.degree. angular change between the reflected light and the incident light because a total internal reflection prism reflects light through two 90.degree. internal angles.
Although the use of mirrors and prisms resulted in devices with improved characteristics those devices tended to be either subject to dimensional changes affecting other resonator cavity components, limited to misalignment insensitivity only along a single axis, or larger in size than can be conveniently incorporated into the compact volume of a resonator cavity. It is, therefore, apparent that the accuracy, reliability and lifetime of a laser transmitter (particularly a Q-switched, mode-locked, optically pumped laser) will be enhanced if the resonator cavity can be made dimensionally stable and insensitive to misalignment.